Production, properties, and uses of fatty acid salts

Thread Content

1. Production method: Since fatty acids are weakly acidic, the hydrogen atoms of the carboxyl group in their molecular structure can undergo substitution reactions with active metals, metal oxides, or hydroxides to form corresponding salts. Different production methods can be used depending on the various applications. 1.1 Precipitation method: Also known as the displacement method or wet production, this is a commonly used industrial method for producing almost all fatty acid salts. The reaction can proceed in two steps. The first step is the reaction of fatty acids with sodium hydroxide solution: Since this is a reversible reaction, a trace amount of free sodium hydroxide must remain at the end of the reaction to ensure the formation of sodium soaps. The second step involves the dissociation of these sodium soaps in water into ROO⁻ and Na⁺ ions, which then undergo substitution reactions with water-soluble metal salts. When saturated fatty acids with medium to long carbon chains react with sodium hydroxide solution, the resulting sodium fatty acid salts form a colloidal solution; as the length of the carbon chain increases, the gelation energy increases, resulting in lower solubility in water. For this reason, the concentration of the sodium hydroxide solution used in the neutralization reaction cannot be high; otherwise, viscous acidic soaps (RCOOH·RCOONa) are formed. Adding ethanol can solubilize the colloid and accelerate the completion of the reaction. This phenomenon should also be taken into account in displacement reactions. In industry, the concentration of sodium hydroxide solution is generally 5–6%, while metal salts are used to prepare solutions with a concentration of 15–20%. Both steps of the reaction take place within the reactor. Production is generally carried out at atmospheric pressure, with the reaction temperature varying depending on the fatty acids and metal salts used. The melting points are above 95°C for barium and calcium stearates, 90°C for zinc stearate, 70–72°C for lead stearate, and 92°C for aluminum stearate. To obtain fatty acid salt products, further processes such as washing and pressing are required to remove Na+, SO42- and most of the water from the stearate salts, followed by drying and separation, grinding, and packaging. Figure 1 is a schematic diagram of a typical process flow for producing stearates. 1.2 Melting method: This method is commonly used when stearates are used as drying agents. Commonly used are oxides, hydroxides, carbonates, or acetates such as lead oxide, barium hydroxide or barium acetate, manganese dioxide, zinc oxide, calcium oxide, and copper oxide or copper carbonate, together with fatty acids or fats, tall oil, or rosin acid; rapid stirring at 180–300°C ensures complete reaction. If the reaction temperature is too low or the reaction viscosity is too high, an oil-soluble solvent can be added for dilution, which helps to prevent the decomposition of the reactants at high temperatures. Hydroxides can react with fatty acids in one step, or the reaction can be carried out with the dispersing effect of surfactants. In the papermaking industry, additives such as fatty acid salts required are prepared as 50% water-containing suspension dispersions for coating high-quality paper, and surfactants with dispersion capabilities need to be added at this time. In the papermaking industry, for the coating of high-quality papers, calcium fatty acid salts are used in a dispersed form, combined with other additives to formulate coating slurries. The melting method uses fewer chemical raw materials than the precipitation method, reducing wastewater. However, the product quality obtained by the precipitation method is excellent. Fatty acid salts produced by the melting method have a high density and are known as heavy fatty acid salts. 1.3 Direct Method The direct method is also known as the semi-wet semi-dry production method. A direct reaction between reactive metal powder and fatty acids is employed. This method is rarely used. RCOOH + M RCOOM + H2: First, dissolve the fatty acid in a solvent; then add the metal powder while stirring continuously, and heat at 100–125°C until all of the metal powder has dissolved. 2. Physicochemical properties of fatty acid salts 2.1 Solubility Except for ammonium, potassium, and sodium soaps of fatty acids, almost all metal salts of fatty acids with medium to long carbon chains are insoluble in water. Metal salts of medium- and high-carbon-chain fatty acids have very low solubility in polar solvents and form gels in non-polar solvents; they can only dissolve in mineral oils and fats. This phenomenon is mainly due to the fact that, as the hydrophobic hydrocarbon chains increase in length, the influence of the hydrophilic carboxyl groups decreases, and moreover, the hydrogen atoms on the carboxyl groups are replaced by highly hydrophobic metals, resulting in poor solubility in water but good solubility in oils. 2.2 Melting Point All fatty acid salts have different crystal forms; therefore, the melting point is one of the important physical properties of fatty acid salts. Table 1 shows the melting points of some metal salts of fatty acids. 2.3 Appearance: Fatty acid salts are mostly white powders, such as zinc salts, calcium salts, barium salts, aluminum salts, lead salts, cadmium salts, magnesium salts, etc. Nickel stearate is a pale green powder, copper stearate is a sky-blue powder, manganese stearate is a pink powder, and ammonium stearate is a yellow waxy solid. 3 Uses: The most commonly used fatty acid salts are calcium salts, accounting for about 60%; zinc salts account for about 20%; the remaining 20% consists of magnesium salts, lead salts, cadmium salts, lithium salts, as well as small amounts of manganese salts, cobalt salts, tin salts, iron salts, silver salts, strontium salts, and nickel salts. Based on their industrial applications, they can be classified as follows: 3.1 Used in the polymer industry – Low-chain saturated fatty acid salts such as octanoates are often used in polymers as heat stabilizers, curing agents, and catalysts. High-carbon-chain saturated fatty acids such as stearates are often used in polymer materials as release agents, activators, stabilizers, lubricants, etc. Table 1 Melting points of some fatty acid metal salts
Cation Heptanoic acid Octanoic acid Nonanoic acid Lauric acid Myristic acid Palmitic acid Stearic acid Oleic acid
Lithium – – – 229.6–229.8 223.6–224.2 224–225 220.5–221.5 –
Sodium 238–239 – – >260 – Decomposition 100 (decomposition)
Calcium – – 216 182–183 – 153–156 150–154 83–84
Magnesium – – – 150.4 131.6 121–122 132 –
Thallium – – – 125–126 120–123 117–155 119 83
Lead 85 100 95–100 104.6–104.8 108.6–108.8 112.2–112.4 115.6–115.8 50
Silver – – – 211–213 211 209 205 –
Mercury 106.5 – – 100 – 105 112.2 102–103
Zinc 131–132 133–135 131–132 128 – 129 130 70
Copper – 264–266 260 111–113 – 115–120 125 100
Nickel – – – 44 – 80 80–86 18–20
Cobalt – – – – 70–75 73–75 – 3.2
Used in various coatings. The main functions of these fatty acid salts are dispersion, water resistance, filling capacity, homogeneity, and phase compatibility. Table 3 lists the various stearates used for different types of coatings. To produce high-quality paper, fillers are required to make the paper surface smooth and pore-free; such fillers contain 50% suspended and dispersed substances, including additives such as calcium stearate. 3.3 Other uses Zinc stearate is used as a non-toxic stabilizer for polyvinyl chloride, as a lubricant and thickener in cosmetics, as a release agent in powder metallurgy and plastic products, and as an activator for vulcanization catalysts in rubber products. Calcium stearate is used as a non-toxic stabilizer for polyvinyl chloride, as well as a lubricant; it serves as a component in paint flateners and water-resistant agents. In the pen manufacturing industry, it is used in the production of lead cores, and it acts as a thickening agent for lubricants or ointments. Lead stearate is used as a stabilizer and lubricant in various transparent soft and rigid polyvinyl chloride products, as a thickener for lubricants, and as a smoothing agent for paints. Barium stearate is used as a heat-stabilizer for polyvinyl chloride, a high-temperature resistant lubricant, and a high-temperature release agent in the rubber industry. Aluminum distearate is suitable as a heat stabilizer for polyvinyl chloride; it is appropriate for opaque products, and due to its low toxicity, it can be used in products that come into contact with food. It is also used as a raw material for metal rust inhibitors, a waterproofing agent for building materials, a brightener for inks, and a thickener for cosmetics. Aluminum stearate is used as a non-toxic heat stabilizer for PVC plasma films, as well as a plastic lubricant. Magnesium stearate is used as a heat stabilizer for polyvinyl chloride, in cosmetic powders, emollient ointments, and as a raw material for pharmaceutical tablets. It serves as a lubricant and brightener in bakelite plastics, and as a transparent matting agent in the paint industry. Cadmium stearate is used as a heat stabilizer for polyvinyl chloride (providing good initial heat resistance), as a coloring agent in the ceramics industry, and as a smoothing and transparentizing softener for high-quality rubber products. Manganese stearate can be used as a catalyst, lubricant, and in cosmetics. Copper stearate is used as a catalyst produced from surfactant tertiary amines, as an antifouling agent in antifouling coatings, for bronzing, etc. Nickel stearate can be used as a catalyst for synthesizing tertiary amines. Ammonium stearate is used as a dyeing and printing additive, as well as in waterproofing agents for cement concrete. In addition, calcium stearate, aluminum, magnesium, zinc salts, and the like can also be used in industries such as powder metals, metal processing, lubricants, drilling, cement products, ceramics, **, and others. Zinc undecylenate soaps have good antibacterial properties; aluminum fatty acid iodides combined with trace amounts of iodine can also exert antibacterial effects when incorporated into cosmetics, soaps, and cleaning products, and are used as disinfectants in hospitals. Silver montanate is used in photosensitive film. Zinc octoate and stannous octoate can be used as curing agents for insulating materials and polyurethane foam plastics, respectively. Fatty dicarboxylic or polycarboxylic silver salts can be used as catalysts for oxidation, cyclization, isomerization, cleavage, and other reactions. This post was last edited by zxh6267 on 2009-3-19 11:59.]